Lubricating oils for cryogenic services, such as refrigerating oils and insulating oils, are desirably formed of base oils having low pour points in the range of -30.degree. to -60.degree. C. Refrigerating oils are also required to have good miscibility with a variety of Freon refrigerants at low temperatures. In particular, base oils of high aromaticity that are capable of efficient dissolution of poorly oil-soluble Freon refrigerants such as R-22 and R-502 are desired.
In addition to good flowability at low temperatures, insulating oils which are used in high electric field are required to have the ability to absorb the hydrogen gas that evolves upon partial discharge and this also gives rise to the need for base oils having high aromaticity. Furthermore, base oils for insulating oils are desired to have extremely low viscosities (e.g., from 3.5 to 6 cSt at 40.degree. C.). For other applications such as solvents for rubbers and metal soaps, base oils having high aromaticity and boiling at temperatures of from about 250.degree. to 500.degree. C. are desired.
Lubricant base oils having the performance characteristics described above have conventionally been produced from naphthene base crudes or synthetic oils with high aromaticity have been used to attain the same purpose. But today naphthene base crudes are no longer easily available and the production of synthetic oils with high aromaticity is costly. Under these circumstances, it is required to establish a technology that is capable of producing the desired lubricant base oils from paraffin base or mixed base crudes but no satisfactory technique has yet been proposed.
As is well known, lubricant base oils are typically produced by combinations of such techniques as hydrofining, solvent extraction, dewaxing with a solvent or by hydrocracking, treatment with activated clay, and sulfuric acid treating. In the step of solvent extraction, a suitable extraction solvent such as furfural is used and the raffinate portion of the feed is recovered to be subjected to subsequent steps of refining. Methods based on the approach are described in Japanese Patent Application (OPI) Nos. 164109/1983, 101804/1985 and 120793/1985 (the term "OPI" as used herein means an unexamined published Japanese patent application) and U.S. Pat. Nos. 3,640,868, 3,617,473, and 3,759,817. A method involving the recovery of the extract portion resulting from solvent extraction is described in Japanese Patent Publication No. 24395/1982; according to this method, the extract is recovered in the first stage of extraction and subjected to the second stage of extraction, from which the raffinate is recovered and passed through the steps of dewaxing and hydrofining. This method is described as being capable of producing oils with high aromaticity. However, the pour point of the product oil, even if it is wax free, is not lower than the temperature to which the raffinate was cooled to cause wax crystallization.
Japanese Patent Publication No. 3321/1976 discloses a process in which the extract oil is hydrofined with an Ni-W catalyst and mixed with a raffinate oil prior to dewaxing. The oil produced by this process does not have high aromaticity and no significant drop in the pour point is attainable by the dewaxing step.
If lubricant fractions are subjected to hydrofining or dewaxing under rigorous conditions, base oils having pour points of from about -10.degree. to -30.degree. C. and n-d-M ring analysis values of from about 4 to 8% C.sub.A are typically obtained but in practical applications it is difficult to produce base oils having much lower pour points and yet high aromaticity from paraffin base or mixed base crudes. Refrigerating oils that are produced from naphthene base crudes and which have viscosities of from 10 to 100 cSt at 40.degree. C. are known to have n-d-M ring analysis values of from about 2 to 14% C.sub.A and pour points of about -35.degree. C.